Ground potential rise sensors

ABSTRACT

Ground Potential Rise (GPR) sensors measure the GPR between the ground grid and the remote earth. A GPR sensor may provide a GPR measurement to an existing distance protection, which may operate based on this measurement automatically. A GPR sensor may comprise a potential transformer configured such that one wire of the high voltage side of the potential transformer is coupled to the ground grid of a substation. The second wire of the potential transformer is coupled to an insulated wire which is coupled to a ground rod or multiple of ground rods bonded together, that is driven into the earth. The low voltage side of the potential transformer is used to connect Distance Relays, Voltage Relays, DFR or Alarms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims the benefit ofU.S. application(s) Ser. No. 13/801,347, filed Mar. 13, 2013, which ishereby incorporated herein by reference in the entirety.

TECHNICAL FIELD

The present invention relates generally to protection of electricalpower systems, and more particularly, some embodiments relate to groundpotential rise sensors for applications in high voltage electricalsubstations.

DESCRIPTION OF THE RELATED ART

An electrical power system usually comprises protection systemsconfigured to protect, monitor, and control various electrical powerequipment and devices. In the event of a fault or short circuit, theseprotection systems may isolate the faulted area from the rest of theelectrical power systems and prevent the fault from affecting a portionor the entire electrical power system, thereby ensuring reliable andstable operations of the electrical power system.

FIG. 1 (prior art) illustrates operations of a prior art distance relay.The illustrated power system 100 comprises buses 101 and 111, voltagesources 102 and 112, relays 103 and 113, potential transformers 104 and114, current transformers 105 and 115, and circuit breakers 106 and 116.The relays 103 and 113, potential transformers 104 and 114, currenttransformers 105 and 115, as well as circuit breakers 106 and 116compose the protection system for the power system 100. In particular,the protection system protects the transmission line 107 and the voltagesources 102 and 112.

Single-line-to-ground faults occur more frequently than other types offaults and ground distance protection relays are commonly used for theprotection of such faults. Distance protection relays operate bydetermining an apparent impedance of the transmission line through themeasurements of the bus voltage and the line current during a fault. Asillustrated, the relays 103 and 113 are distance relays that require acurrent transformer (CT) and a potential transformer (PT) as primarysensing inputs. PT 104 measures voltage of the bus 101 and PT 114measures voltage of the bus 111. The measured voltages are withreference to the substation ground which is the ground grid of thesubstation. CT 105 and CT 115 measure the current flowing through thetransmission line 107.

An apparent impedance (Za) of the transmission line 107 is calculated byusing measurements provided by the CTs 105 and 115 and the PTs 104 and114. Upon determining Za is equal to or less than a predeterminedimpedance value, the relays 103 and 113 may trip open the circuitbreakers 106 and 116 by sending them a signal, respectively. Zone 1 ofthe ground distance relay is usually set as a percentage (for example,80-90%) of the line impedance.

The relays 103 and 113 calculate the apparent impedance Za according toEquation (1):

$\begin{matrix}{Z_{a} = \frac{V_{LG}}{I_{ph} + {K\; 3\; I_{0}}}} & (1)\end{matrix}$where Za is the apparent impedance as seen by the relay; V_(LG) is theLine-to-Ground voltage for each of the three phases; I_(ph) is the phasecurrent for the fault involved; K is the line compensation factor (i.e.,

$\frac{\left( {Z_{0} - Z_{1}} \right)}{3\; Z_{1}}$where Z₀ is the zero sequence impedance of the line and Z₁ is thepositive sequence impedance of the line); and 3I₀ is the residualcurrent flowing in the transmission line where the fault occurs.

BRIEF SUMMARY OF THE INVENTION

According to various embodiments of the invention, systems and methodsfor adaptive distance protection is provided. The adaptive distanceprotection system and method measure the potential of a system ground inreference to the remote earth to take into account of the groundresistance in calculating the apparent impedance of a transmission line.Conventional protection systems usually disregard the effect of theground resistance between the ground grid and true remote earth. Voltagemeasures that are in reference to the ground grid are assumed to be inreference to the true remote earth. Even though the ground resistance isgenerally small, ignoring the effect of the ground resistance may resultin an improper calculation of the apparent impedance and misoperation ofthe transmission line.

GPR sensors measure the GPR between the ground grid and the remoteearth. A GPR sensor may provide a GPR measurement to an existingdistance protection, which may operate based on this measurementautomatically. For example, distance relays or voltage relays that usevoltage as one of the operating quantity may use the measurementprovided by the GPR sensor. Accordingly, accuracy of the relays and thetripping logic are improved, and potential mis-operations of the relaysare reduced or eliminated. The fault location logic may also be improvedas a result.

In various embodiments, the adaptive distance protection system maycomprise a potential transformer measuring the ground potential rise(GPR) between the ground grid and the true remote earth. An accurateassessment of the apparent impedance of a transmission line is thereforeprovided and reduces the risk of misoperation. Various embodiments maycalculate the apparent impedance of a transmission line based on theline current, the line voltage, and the GPR measured.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments of the invention. Thesedrawings are provided to facilitate the reader's understanding of theinvention and shall not be considered limiting of the breadth, scope, orapplicability of the invention. It should be noted that for clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 illustrates operations of a prior art distance relay.

FIG. 2A illustrates an exemplary adaptive distance protection systeminstalled in a power system in accordance with an embodiment.

FIG. 2B illustrates a block diagram illustrating operation of anadaptive distance protection system in accordance with an embodiment.

FIG. 2C illustrates a sequence diagram for conducting the symmetricalcomponent analysis of the system 200 installed with an exemplaryadaptive distance protection system.

FIG. 3A illustrates an installation of an exemplary adaptive distanceprotection system in accordance with an embodiment.

FIG. 3B illustrates an installation of an exemplary adaptive distanceprotection system in accordance with an embodiment.

FIG. 3C illustrates an exemplary installation of an insulated wire to aremote location in accordance with an embodiment.

FIG. 3D illustrates an exemplary installation of a GPR sensor inside acontrol house in accordance with an embodiment.

FIG. 4 is a flow chart illustrating an exemplary method of adaptivedistance protection in accordance with an embodiment.

FIG. 5 illustrates an example computing module that may be used inimplementing various features of embodiments of the invention.

The figures are not intended to be exhaustive or to limit the inventionto the precise form disclosed. It should be understood that theinvention can be practiced with modification and alteration, and thatthe invention be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed toward a system and method forproviding an adaptive distance protection. Various embodiments may beinstalled in a power system to provide an accurate measurement of theapparent impedance of the transmission lines thereby ensuring reliableand stable operations of the power system. Some embodiments may augmentexisting protection systems installed in a power system, for example,via a retrofitting process, to enhance reliability of the power system,reduce costs and expand opportunities for deployment.

Conventional ground distance relays are prone to misoperate in the eventof a fault as a result of assuming voltage measurements that are inreference to the ground grid are in reference to the true remote earth,since the phase voltage and the line current measured at the bus areused to determine the apparent impedance of a transmission line. Whencalculating impedance of the transmission in the event of a fault, thevoltage is assumed to be with reference to true ground, i.e. no groundresistance exists between the grid ground and the remote earth. However,the relays at the substation measure the voltage with respect to theground grid of the substation and not to the true ground. This mayresult in inaccurate measurements of phase voltages at the bus becausethe ground resistance between the ground grid and true remote earth isnot always negligible.

As illustrated in FIG. 1, the calculation of the apparent impedance ofthe transmission line 107 may be based on the voltage measurementsprovided by the potential transformers 104 and 114. Misoperation of thetransmission line 107 may occur as the calculated apparent impedance Zadoes not reflect the actual operation (i.e., the actual impedance) ofthe transmission line. For example, a V_(LG) of 20 V may be measured as15 V during a fault event, which results in a calculation of Za at 7.5Ωeven though the actual line impedance is 10Ω. As a result, line 107 maybe tripped open mistakenly.

The adaptive distance protection system and method measure the potentialof a system ground in reference to the remote earth to take into accountof the ground resistance in calculating the apparent impedance of atransmission line. In various embodiments, the adaptive distanceprotection system may comprise a potential transformer measuring theground potential rise (GPR) between the ground grid and the true remoteearth. An accurate assessment of the apparent impedance of atransmission line is therefore provided and reduces the risk ofmisoperation. Various embodiments may calculate the apparent impedanceof a transmission line based on the line current, the line voltage, andthe GPR measured. Consequently, the fault location may also bedetermined correctly.

FIG. 2A and FIG. 2B illustrate operation of an adaptive distanceprotection system in accordance with an embodiment. FIG. 2A illustratesan exemplary adaptive distance protection system installed in a powersystem 200 in accordance with an embodiment. The electrical power system200 comprises buses 201 and 202 connected by a transmission line 209,voltage sources 207-208, a current transformer 210, potentialtransformers 213-214, and relays 211-212. The current transformer 210,relays 211-212, and phase transformers 213-214 compose the protectionsystem of the power system 200. The resistors 215 and 216 illustrate theground grid resistance between the ground grid 203-204 of the powersystem 200 and the true remote earth 205.

The relay 211 may comprise a directional element that determines thedirection of a fault. One or more current and/or voltage signals by CTsand PTs including CT 210 and PTs 213-214 may be provided to thedirectional element for determining the direction of a fault.

Existing relays may be augmented to reduce the risk of misoperation andto enhance reliability via retrofitting. In one embodiment, the relay212 may be an existing relay of the power system 200, and the relay 211and the PT 213 are deployed. As illustrated, the existing relay 212 maybe coupled to the relay 211. As such, GPR is included in the operationof the existing relay 212.

CT 210 measures the current of the transmission line 209, PT 214 and PT213 measure the voltage of the bus 202. These measurements may beprovided to the relays 211-212. In the event of a line-to-ground faulton bus 201, the apparent impedance is calculated according to Equation(2):

$\begin{matrix}{Z_{a} = \frac{V_{LG} \pm {I_{gg}*R_{gg}}}{I_{ph} + {K\; 3I_{0}}}} & (2)\end{matrix}$where Za is the apparent impedance as seen by the relay; I_(gg) is theground current returning to the ground grid; R_(gg) is the ground gridresistance, V_(LG) is the Line-to-ground voltage for each of the threephases I_(ph) is the phase current for the fault involved; K is the linecompensation factor equal to (Z0-Z1)/3Z1, where Z₀ is the zero sequenceimpedance of the line and Z₁ is the positive sequence impedance of theline; and 3I₀ is the residual current flowing in the faulted line.

Ground Potential Rise (GPR) is included into the calculation of theapparent impedance. Voltage measured by the PT 214 is used as areference voltage. When the GPR is positive, the fault current flowsfrom the grid grounds 203 and 204 to the remote earth ground 205. Whenthe GPR is negative, the fault current flows from the remote earthground 205 to the grid grounds 203 and 204. The GPR is compared to thevoltage measured by the PT 214, in particular, the phase of the GPR iscompared to the phase of the voltage measured by the PT 214, todetermine a voltage (V_(LG)±I_(gg)*R_(gg)) for calculating the apparentimpedance Za.

In addition, fault location may be determined according to Equation (3):

$\begin{matrix}{{nXa} = {{{n({Im})}{Za}} = {{n({Im})}\frac{V_{LG} \pm {I_{gg}*R_{gg}}}{I_{ph} + {K\; 3\; I_{0}}}}}} & (3)\end{matrix}$where Xa is the apparent reactance, which is the imaginery part of theapparent impedance Za, n is the per unit distance to the fault (i.e.,the distance to the fault divided by the total length of thetransmission line).

FIG. 2B illustrates a block diagram illustrating operation of anadaptive distance protection system in accordance with an embodiment.Various embodiments may compare the phase of the GPR to the phase of thereference voltage to determine the proper voltage for calculating theapparent impedance Za. The reference voltage and GPR are provided to thecomparators 230-232. When the comparator 230 determines that GPR is inphase with the reference voltage, GPR is subtracted from the referencevoltage resulting in an output voltage V_(LG)−I_(gg)*R_(gg) used forcalculating the apparent impedance Za. When the comparator 231determines that GPR is out of phase with the reference voltage, GPR isadded to the reference voltage resulting in an output voltageV_(LG)+I_(gg)*R_(gg) used for calculating the apparent impedance Za.When the comparator 232 determines that there is no GPR (i.e., GPR iszero), the output voltage is the reference voltage V_(LG) forcalculating the apparent impedance Za.

FIG. 2C illustrates a sequence diagram for conducting the symmetricalcomponent analysis of the system 200 installed with an exemplaryadaptive distance protection system. As illustrated, the zero sequenceof the power system 200 is coupled to the remote earth. Voltage of theneutral bus may be calculated accurately to reflect the GPR, which maybe measured by the PT 213.

FIG. 3A illustrates an installation of an exemplary adaptive distanceprotection system in accordance with an embodiment. As illustrated,transmission lines 307-309 are coupled to a station 301 and supported bytransmission towers 310-312. Each transmission line may be installedwith a potential transformer for measuring phase voltages. In theillustrated example, the transmission line 309 experiences aline-to-ground fault. The PT 303 installed on the transmission line 309measures voltage of the transmission line 309. The PT 303 is coupled tothe ground grid 305. Further, a PT 304 is installed to measure thepotential between the ground grid 305 and the remote earth 306. Invarious embodiments, the PT 304 may be a 12 kV/120V transformer.

As illustrated, the GPR sensing PT 304 is installed such that one sideis coupled to the substation ground grid and the other side is connectedto an insulated wire 320. The insulated wire 320 is insulated from anystructures and extend to a distance whereas the electrical zone ofinfluence no longer exists from nearby structures. In variousembodiments, this distance may be hundreds to thousands of feet. Thisinsulated wire 320 should be coupled to a ground rod 321 located faraway from the substation and made of electrical conductive metals suchas copper. The ground rod 321 is driven into the earth at a certaindepth (for example, at least 15 to 20 feet). The insulated wire 320 maybe of any size. In some embodiments, the insulated wire 320 may becoupled to multiple ground rods that are bonded together.

FIG. 3B illustrates an installation of an exemplary adaptive distanceprotection system in accordance with an embodiment. Similar to theinstallation illustrated in FIG. 3A, one side of the GPR sensing PT 304is coupled to the substation ground grid. The other side of the GPRsensing PT 304 is coupled to an insulated wire 325. The insulated wire325 is buried underground to a distance from the substation where noelectrical zone of influence exists and connected at this location to aground rod 326 driven down into the earth.

FIG. 3C illustrates an exemplary installation of an insulated wire 352to a remote location in accordance with an embodiment. The remotelocation is coupled to a ground rod 351, which is used as a remote earthzero voltage reference. As illustrated, the ground rod 351 is installedbelow the ground surface 350. In various embodiments, the ground rod 351is at least 10 to 20 feet long. The ground rod 351 may be made ofmaterials such as steel or copper-clad steel. In various embodiments,the bonded wire 352 that is coupled to the ground rod 351 is at least 2feet below the ground surface 350. One end of the wire 352 is coupled tothe ground rod 351, and the other end of the wire 352 is coupled to asubstation. The ground rod 351 is located at a remote location that isfar away (e.g., at least 3-4 thousand feet) from the substation. Invarious embodiments, the wire 352 is an insulated wire (e.g., aninsulated wire No. 4 with minimum 5-12 kV insulation). In oneembodiment, the wire 352 may be surrounded by a pipe 353 of differentsizes, such as a PVC pipe, schedule 80, 1.5-2″.

In various embodiments, a surge arrester may be connected on the highvoltage side of a GPR sensor (e.g., a GPR sensing PT.) The surgearrester may protect against any high voltage spikes coming from theremote earth wire, protecting the GPR sensor from possible insulationdamage. The low voltage side of the GPR sensor may be connected toprotective relay's Digital Fault Recording (DFR) and could also alarmfor such conditions. FIG. 3D illustrates an exemplary installation of aGPR sensor inside a control house in accordance with an embodiment. AGPR sensor may be coupled to an electrical steel box 361 via a wire 360to the ground connection 362. The DFR inside the control room is alsocoupled via connection 363 to the substation where the GPR sensor isinstalled. Various monitoring equipment may be coupled to the controlhouse. For example, as illustrated, the connection 364 is coupled to avoltage relay, the connection 365 is coupled to a relay ground distance,and the connection 366 is coupled to other monitoring equipment.

FIG. 4 is a flow chart illustrating an exemplary method 400 of adaptivedistance protection in accordance with an embodiment. At step 402, aline current, a phase voltage V_(REF) with reference to the grid ground,and GPR are measured with reference to the remote earth. At step 404,GPR and V_(REF) are compared, in particular, phase angles of GPR andV_(REF) are compared to determine whether GPR is in phase or out ofphase with V_(REF). At step 406, upon determining that GPR is in phasewith V_(REF), the output voltage for calculating the apparent impedanceis determined as V_(REF)−GPR. At step 408, upon determining that GPR isout of phase with V_(REF), the output voltage for calculating theapparent impedance is determined as V_(REF)+GPR.

At step 410, the apparent impedance of the transmission line iscalculated based on the line current and the output voltage. In variousembodiments, the apparent impedance of the transmission line iscalculated according to Equation (2). At step 412, the calculatedapparent impedance of the transmission line is compared to the referenceimpedance. The reference impedance may be a percentage of the lineimpedance of the transmission line. In one embodiment, the referenceimpedance value is between 80% and 90% of the line impedance. At step414, the transmission line is tripped open when the calculated apparentimpedance is determined to be equal to or less than the referenceimpedance value. In one embodiment, an instruction signal is generatedand transmitted to open a circuit breaker in response to a determinationthat the calculated apparent impedance is equal to or less than thereference impedance.

As used herein, the terms less than, less than or equal to, greaterthan, and greater than or equal to, may be used herein to describe therelations between various objects or members of ordered sets orsequences; these terms will be understood to refer to any appropriateordering relation applicable to the objects being ordered.

As used herein, the term module might describe a given unit offunctionality that can be performed in accordance with one or moreembodiments of the present invention. As used herein, a module might beimplemented utilizing any form of hardware, software, or a combinationthereof. For example, one or more processors, controllers, ASICs, PLAs,PALs, CPLDs, FPGAs, logical components, software routines or othermechanisms might be implemented to make up a module. In implementation,the various modules described herein might be implemented as discretemodules or the functions and features described can be shared in part orin total among one or more modules. In other words, as would be apparentto one of ordinary skill in the art after reading this description, thevarious features and functionality described herein may be implementedin any given application and can be implemented in one or more separateor shared modules in various combinations and permutations. Even thoughvarious features or elements of functionality may be individuallydescribed or claimed as separate modules, one of ordinary skill in theart will understand that these features and functionality can be sharedamong one or more common software and hardware elements, and suchdescription shall not require or imply that separate hardware orsoftware components are used to implement such features orfunctionality.

Where components or modules of the invention are implemented in whole orin part using software, in one embodiment, these software elements canbe implemented to operate with a computing or processing module capableof carrying out the functionality described with respect thereto. Onesuch example computing module is shown in FIG. 5. Various embodimentsare described in terms of this example—computing module 500. Afterreading this description, it will become apparent to a person skilled inthe relevant art how to implement the invention using other computingmodules or architectures.

Referring now to FIG. 5, computing module 500 may represent, forexample, computing or processing capabilities found within desktop,laptop and notebook computers; hand-held computing devices (PDA's, smartphones, cell phones, palmtops, etc.); mainframes, supercomputers,workstations or servers; or any other type of special-purpose orgeneral-purpose computing devices as may be desirable or appropriate fora given application or environment. Computing module 500 might alsorepresent computing capabilities embedded within or otherwise availableto a given device. For example, a computing module might be found inother electronic devices such as, for example, digital cameras,navigation systems, cellular telephones, portable computing devices,modems, routers, WAPs, terminals and other electronic devices that mightinclude some form of processing capability.

Computing module 500 might include, for example, one or more processors,controllers, control modules, or other processing devices, such as aprocessor 504. Processor 504 might be implemented using ageneral-purpose or special-purpose processing engine such as, forexample, a microprocessor, controller, or other control logic. In theillustrated example, processor 504 is connected to a bus 502, althoughany communication medium can be used to facilitate interaction withother components of computing module 500 or to communicate externally.

Computing module 500 might also include one or more memory modules,simply referred to herein as main memory 508. For example, preferablyrandom access memory (RAM) or other dynamic memory, might be used forstoring information and instructions to be executed by processor 504.Main memory 508 might also be used for storing temporary variables orother intermediate information during execution of instructions to beexecuted by processor 504. Computing module 500 might likewise include aread only memory (“ROM”) or other static storage device coupled to bus502 for storing static information and instructions for processor 504.

The computing module 500 might also include one or more various forms ofinformation storage mechanism 510, which might include, for example, amedia drive 512 and a storage unit interface 520. The media drive 512might include a drive or other mechanism to support fixed or removablestorage media 514. For example, a hard disk drive, a floppy disk drive,a magnetic tape drive, an optical disk drive, a CD or DVD drive (R orRW), or other removable or fixed media drive might be provided.Accordingly, storage media 514 might include, for example, a hard disk,a floppy disk, magnetic tape, cartridge, optical disk, a CD or DVD, orother fixed or removable medium that is read by, written to or accessedby media drive 512. As these examples illustrate, the storage media 514can include a computer usable storage medium having stored thereincomputer software or data.

In alternative embodiments, information storage mechanism 510 mightinclude other similar instrumentalities for allowing computer programsor other instructions or data to be loaded into computing module 500.Such instrumentalities might include, for example, a fixed or removablestorage unit 522 and an interface 520. Examples of such storage units522 and interfaces 520 can include a program cartridge and cartridgeinterface, a removable memory (for example, a flash memory or otherremovable memory module) and memory slot, a PCMCIA slot and card, andother fixed or removable storage units 522 and interfaces 520 that allowsoftware and data to be transferred from the storage unit 522 tocomputing module 500.

Computing module 500 might also include a communications interface 524.Communications interface 524 might be used to allow software and data tobe transferred between computing module 500 and external devices.Examples of communications interface 524 might include a modem orsoftmodem, a network interface (such as an Ethernet, network interfacecard, WiMedia, IEEE 802.XX or other interface), a communications port(such as for example, a USB port, IR port, RS232 port Bluetooth®interface, or other port), or other communications interface. Softwareand data transferred via communications interface 524 might typically becarried on signals, which can be electronic, electromagnetic (whichincludes optical) or other signals capable of being exchanged by a givencommunications interface 524. These signals might be provided tocommunications interface 524 via a channel 528. This channel 528 mightcarry signals and might be implemented using a wired or wirelesscommunication medium. Some examples of a channel might include a phoneline, a cellular link, an RF link, an optical link, a network interface,a local or wide area network, and other wired or wireless communicationschannels.

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as, forexample, memory 508, storage unit 520, media 514, and channel 528. Theseand other various forms of computer program media or computer usablemedia may be involved in carrying one or more sequences of one or moreinstructions to a processing device for execution. Such instructionsembodied on the medium, are generally referred to as “computer programcode” or a “computer program product” (which may be grouped in the formof computer programs or other groupings). When executed, suchinstructions might enable the computing module 500 to perform featuresor functions of the present invention as discussed herein.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not of limitation. Likewise, the various diagrams maydepict an example architectural or other configuration for theinvention, which is done to aid in understanding the features andfunctionality that can be included in the invention. The invention isnot restricted to the illustrated example architectures orconfigurations, but the desired features can be implemented using avariety of alternative architectures and configurations. Indeed, it willbe apparent to one of skill in the art how alternative functional,logical or physical partitioning and configurations can be implementedto implement the desired features of the present invention. Also, amultitude of different constituent module names other than thosedepicted herein can be applied to the various partitions. Additionally,with regard to flow diagrams, operational descriptions and methodclaims, the order in which the steps are presented herein shall notmandate that various embodiments be implemented to perform the recitedfunctionality in the same order unless the context dictates otherwise.

Although the invention is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

What is claimed is:
 1. A ground potential rise sensor configured tomeasure ground potential rise of a substation, comprising: a firstpotential transformer, a first side of the first potential transformercoupled to the substation ground grid; a second potential transformer, afirst side of the second potential transformer coupled to the substationground grid; a first potential transformer, a second side of the firstpotential transformer coupled to a first end of a first wire; a secondpotential transformer, a second side of the second potential transformercoupled to a first end of a second wire; the first wire, the second endof the first wire coupled to a first ground rod; the second wire, thesecond end of the second wire coupled to a second ground rod; whereinthe first potential transformer measures a first voltage and the secondpotential transformer measures a second voltage, further comprisingdetermining the ground potential rise to be the first voltage; andwherein a proper voltage for calculating an apparent impedance isobtained by comparing a phase of the ground potential rise to a phase ofthe second voltage.
 2. The ground potential rise sensor of claim 1,wherein the first wire is insulated.
 3. The ground potential rise sensorof claim 1, further comprising the first ground rod, the first groundrod driven into the earth.
 4. The ground potential rise sensor of claim3, wherein the second end of the first wire and the first ground rod arefree from an electrical zone of influence.
 5. The ground potential risesensor of claim 1, further comprising a surge arrester coupled to thefirst side of the first potential transformer.
 6. The ground potentialrise sensor of claim 1, wherein the second side of the first potentialtransformer is further coupled to a digital fault recorder.
 7. Theground potential rise sensor of claim 1, wherein the second end of thefirst wire is further coupled to a third ground rod, the first groundrod and the third ground rod bonded together.
 8. A method of installinga ground potential rise sensor configured to measure ground potentialrise of a substation, comprising: coupling a first side of a firstpotential transformer to the substation ground grid; coupling a firstside of a second potential transformer to the substation ground grid;coupling a second side of the first potential transformer to a first endof a first wire; coupling a second side of the second potentialtransformer to a first end of a second wire; coupling a second end ofthe first wire to a first ground rod; coupling a second end of thesecond wire to a second ground rod; obtaining a first voltage measuredby the first potential transformer; obtaining a second voltage measuredby the second potential transformer; determining the ground potentialrise to be the first voltage; and obtaining a proper voltage forcalculating an apparent impedance by comparing a phase of the groundpotential rise to a phase of the second voltage.
 9. The method of claim8, wherein the first wire is insulated.
 10. The method of claim 8,further comprising driving the first ground rod into the earth.
 11. Themethod of claim 9, wherein the second end of the first wire and thefirst ground rod are free from an electrical zone of influence.
 12. Themethod of claim 8, further comprising coupling a surge arrester to thefirst side of the first potential transformer.
 13. The method of claim8, further comprising coupling a digital fault recorder to a second sideof the first potential transformer.
 14. The method of claim 8, whereinthe second end of first the wire is further coupled to a third groundrod, the first ground rod and the third ground rod bonded together.